Hydrological Response of the Geul Catchment to the Rainfall in July 2021

Flash floods are a major concern in hydrology and natural hazard science due to their high potential of causing fatalities and damages. They are caused by short, high-intensity rainfall and characterized by a flood event happening within six hours of the onset of precipitation.
In July 2021 an extremely large rainfall event took place in the mid mountain area of Belgium (Ardennes), Germany (Eifel), Luxembourg and the Netherlands (South Limburg). Precipitation amounts of up to 250 mm in 48 hours were recorded in the core of the rainfall area in Belgium. In the Netherlands the Geul catchment was affected the most as the high precipitation resulted in a flash flood hitting the villages and cities along the rivers.

This research is aimed at understanding the hydrological response of the Geul catchment to this extreme rainfall event. To begin with, the catchment and its hydrological behaviour is studied in detail, followed by an event analysis including rainfall, discharge and groundwater data. Furthermore, the antecedent wetness conditions prior to the event are assessed. Next to the data analysis, a wflow model was set up and adjusted to reproduce the event, to evaluate the event contributions of the different tributaries, and to test the impact of different antecedent conditions, rainfall amounts and infiltration capacities on the discharge response. By adjusting three parameters in the model (soil thickness, maximum leakage and a horizontal conductivity parameter), the model was able to reproduce the floods and the different responses of the subcatchments.

In July 2021 the Geul catchment received on average 128 mm precipitation in 48 hours. The highest rainfall sums were recorded in the Belgian part of the catchment and around Ubachsberg. The high rainfall quantities led to high discharges in the catchment. Based on model results, the peak discharge in the central part of the catchment at Gulpen was approximately 124 m3/s. The discharge response of the catchment differed strongly between the subcatchments, especially between the tributaries and the Belgian upstream part of the catchment. The Belgian part contributed about 60% to the flow in Gulpen,
although its contributing area is only 45%. The event run-off coefficient in the Belgian area was approximately 41%, while the event runoff coefficients of the tributaries Gulp and Eyserbeek were 21 and 23%, respectively. The differences in hydrologic responses within the catchment can be linked back to the geology. While the upstream part is characterized by thin soils and impermeable carboniferous rocks at the riverbed, the central and lower part of the catchment consist of a thick unsaturated chalk layer acting as major storage areas. In general, the overall run-off coefficient of 32% is fairly small for such an extreme event. The floodplains and the thick unsaturated zone at the chalk plateaus stored much water, and considerably delayed and dampened the peak. Based on the simulation approximately 75mm of water was stored in the catchment area upstream of Gulpen. 30-days prior to the event the catchment received 50% more rain than the long-term average. Assuming average July wetness conditions, according to the simulation the event peak and cumulative discharges would have been 20-35% lower, and an additional 4.8 x 106 m3 water could have been stored in the catchment upstream of Valkenburg. Consequently, the wetter-than-usual antecedent wetness conditions increased the severity of the flood from being minor (under normal conditions) to the major flooding which was experienced in the downstream catchment areas. However due to climate change, wetter summers are projected to become more frequent in the future and consequently the role of antecedent wetness will become even more important.

In the Geul catchment the geology is a dominant control on the runoff response of the (sub)catchments. The impact of flood mitigation measures can be maximized by taking the role of geology and its interaction with the effect of land-use changes into account in the selection process. Flood mitigation measures are especially required in the Belgian upstream part of the catchment. Retaining water already there, will attenuate and delay peak discharges at the villages and cities downstream.

The final report can be downloaded here.